Electrical power converters are devices for processing electrical power from one form, such as an AC or unregulated DC voltage, into another form, such as one or more regulated DC output voltages. One conventional type of electrical power converter that produces a regulated output voltage is a switching power supply, also commonly referred to as a switch mode power supply or a switched power supply.
Conventional switching power supplies commonly include a power transformer and one or more power switches for alternately coupling an unregulated DC or rectified AC voltage across a primary winding of the power transformer in a series of voltage pulses. These pulses are converted into a series of voltage pulses across one or more secondary windings of the power transformer and then rectified and filtered to provide one or more output DC voltages. The output voltage or voltages of the power converter are commonly regulated by controlling the relative amount of time that the power switch is on (i.e., the duty cycle).
One common type of switching power supply is the flyback power converter, also referred to as an energy storage converter. A flyback power converter works by cyclically storing energy in the power transformer, and then dumping this stored energy into a load. By varying the amount of energy stored and dumped per cycle, the output power can be controlled and regulated. A high power switching transistor connected in series with the primary winding of the power transformer normally provides such a switching function. That is, the on-time and off-time of this power switch controls the amount of energy coupled across the power transformer. When the power switch is on, current flows through the primary winding of the power transformer, and energy is stored in the transformer. When the power switch is off, the stored energy is transferred out into a secondary circuit by means of current flowing out of one or more secondary windings of the power transformer. Note that the secondary current does not flow in the power transformer at the same time that the power switch is on and the primary current is flowing. The reason for this is that in a conventional flyback power converter, a rectifier is coupled to the secondary winding to prevent conduction of current in the secondary winding when the power switch is on.
More specifically, at the beginning of each switching cycle of a conventional flyback power converter, the power switch turns on and couples an input voltage across the primary winding such that current in the primary winding ramps up from zero, thereby storing magnetic energy in the power transformer. The period of time during which the power switch is on is referred to as the drive cycle or drive period. After the power switch is turned off, current through the primary winding is sharply reduced and the voltage across the transformer windings reverses. Reversing the voltage across the secondary winding forward biases the secondary-side rectifier and allows current to be conducted through the secondary winding, thereby releasing the energy stored in the power transformer. This secondary current initially quickly reaches a relatively high value and then decreases over time as energy from the transformer is released. The voltage across the secondary winding initially reaches a high reverse value and decreases slowly during the flyback cycle. The energy from the transformer is coupled to and stored on an output capacitor to produce the desired output voltage. The period of time during which energy is released from the secondary winding is referred to as the flyback cycle or flyback period.
There are two main types of flyback converters. In most conventional flyback converters, the energy stored in the transformer is totally coupled to the output load before the next drive cycle, generally resulting in the secondary current reaching zero during the flyback cycle. Such flyback converters generally are referred to as discontinuous flyback converters. By contrast, in continuous flyback converters, the next drive cycle begins before all stored magnetic energy is released from the transformer, and therefore before the secondary current reaches zero. Discontinuous flyback convertors are more common than continuous flyback converters because relatively simple control circuitry can be used to maintain output voltage regulation by varying the frequency and/or on-time of the power switch to accommodate heavy or light load conditions.
Flyback power converters are advantageous at lower power levels over other switching power converters due to the fact that they are generally simpler, they require a reduced number of components, and they allow multiple regulated outputs to be available from a single supply. Common applications for flyback converters are AC adapters, which may, for example, deliver an output voltage in the range of between 9 VDC to 24 VDC at power levels of 20 to 50 Watts, drawing power from a rectified AC mains, which may vary between 85 VAC to 270 VAC. One reason flyback converters are preferred to other converters for these applications is that they do not require an output choke under these voltage and power conditions.
Conventional flyback converters generally are not used at high power levels, especially at high switching frequencies, because they have many disadvantages that become particularly troublesome under such conditions. In fact, flyback converters are rarely used at power levels exceeding 100 or 200 Watts.
A first disadvantage of using a flyback power converter in high power applications is that flyback converters for such applications are often undesirably bulky. This is due to the need to use bulky transformers, which are typically necessary to store the high amounts of energy that must be transferred from the primary to the secondary of the transformer. One way to allow for smaller transformers is to incorporate an air gap of some appreciable size into the transformer core. The air gap allows greater current capability in the transformer before saturation of the core and, therefore, increases the energy-storage capabilities of a given sized transformer. An air gap, however, generally results in a relatively large leakage inductance, which causes a number of well-known problems including reduced power conversion efficiency. The size of the transformer can also be reduced by operating the flyback converter at high power switching frequencies, so that current conducted through the transformer windings is less likely to reach a level sufficient to cause saturation. As is well known, however, high switching frequencies (e.g., 200 KHz to 2 MHz) can lead to excessive power dissipation in the power switch.
A second disadvantage of using flyback power converters at high power levels is that flyback converters generally have poor power conversion efficiency and high power dissipation under such circumstances. As noted above, the relatively large leakage inductance caused by the air gap in the transformer core is one factor that decreases the power conversion efficiency of the flyback in comparison to other converters. When the power switch is turned on, energy is stored in both the transformer's core (the magnetizing inductance) and in the leakage inductance. When the power switch is turned off, the energy in the core (magnetizing inductance) is coupled to the secondary circuit, but the energy stored in the leakage inductance rings with the capacitance of the power switch, and is conventionally dissipated in a voltage clamping or "snubber" circuit connected to the power switch. In a discontinuous mode flyback converter, this dissipated energy can easily be as much as ten percent (10%) of the energy transferred to the secondary circuit when using a safety isolated transformer.
Another tactor that decreases the power conversion efficiency is the relatively large root-mean-square (RMS) current that flows in the secondary winding during the flyback period. As is known in the art, the current through the secondary winding during the flyback period has a waveform that begins at a value substantially equal to the magnetizing current flowing in the primary winding at the end of the drive period times the transformer turns ratio and decreases substantially linearly to a lower value (in a discontinuous converter the secondary current falls to zero). Because the current waveform has a high initial current value, it has a relatively high RMS value in comparison to a current waveform with lower peak current transferring the same amount of energy. As is known in the art, the resistive losses (I.sup.2 R) in the secondary winding and secondary circuit are proportional to the square of the RMS current. Discontinuous flyback converters result in particularly high power losses because, for a given output power, the initial peaks must be much higher than the current peaks necessary in continuous flyback converters.
A further factor that decreases the power conversion efficiency of a conventional flyback converter are power dissipation losses that occur in the power switch and other semiconductor components when they are switched. Power switches commonly are metal-oxide semiconductor field-effect transistors (MOSFETs or FETs), although other types of transistors (such as bipolar junction power transistors, BJTs) are sometimes used. FETs are preferred because they can accommodate higher switching frequencies than most bipolar power transistors. However, a measurable amount of power is dissipated in the FET when it is turned on, because the drain voltage takes a finite time to decrease to near zero while drain current is flowing. The drain voltage starts at a value of at least the input voltage. If there is a reverse voltage across the transformer at the beginning of the drive cycle, the drain voltage value is higher than the input voltage. Similarly, at the beginning of each flyback cycle, the secondary-side rectifier becomes forward biased and high current begins to flow immediately, resulting in high power losses during the forward recovery time. Finally, at the beginning of each drive cycle in continuous flyback converters, turning on the power switch causes the secondary-side rectifier to quickly become reverse biased, with very high rates of change in the voltage across and current through the secondary-side rectifier during the reverse recovery period. As is well-known, this fast changing current and voltage during the reverse recovery period results in high reverse recovery power losses in the rectifier. It also results in the generation of unwanted noise which may exceed EMI standards or otherwise cause disruption in other circuits or devices.
In prior art discontinuous flyback converters, turn-on losses have been reduced by implementing "zero voltage switching," that is, by reducing the drain voltage of the FET to approximately zero before it is turned on. It is possible to implement zero voltage switching in discontinuous flyback converters because there is sufficient time to reduce the voltage across the FET to zero during the relatively long flyback cycle. For example, a conventional active clamp can be placed in parallel with the primary winding to create a resonant circuit. Energy stored in the leakage inductance is transferred to the clamp capacitor and back to the primary winding during the flyback cycle, causing a fluctuation in the voltage across the power switch. The circuit is timed such that the voltage across the power switch is zero at the time the flyback cycle concludes. Reduction of this voltage across the power switch is enabled in discontinuous flyback converters because the reverse voltage across the primary winding during the flyback cycle naturally falls to a low value when all magnetic energy has been released from the transformer at the end of the flyback cycle Zero voltage switching, however, does not alleviate the problem of power dissipation due to high current peaks in the secondary winding.
A third disadvantage of using discontinuous flyback converters in high power applications is that adverse effects result from the high peak currents, discussed above, and from the high secondary voltage spike which are generated at the beginning of the flyback period due to the high leakage inductance. The high peak currents may damage many semiconductor components and capacitors and, thus, design choices for these components are limited. The secondary voltage spike results in output voltage noise which must be filtered by a bulky choke in order for the converter to meet conventional output voltage noise specifications. In fact, under high power conditions, the output voltage waveform of a flyback often has undesirably high harmonic content that must be filtered even if the leakage inductance produced spike is not considered.
The present invention is directed toward improving the performance of flyback power converters in each of the above areas.